parse-location.js

var tape = require('tape')

var bitfield = new Buffer(require('./fixtures/location-bitfield.json'))

tape.only('should parse location bitfield', function(t) {
  var parsed = parse(bitfield)
  var expected = {
    deltaHeading: 0,
    x: 10936,
    y: 7856,
    animation: 0,
    z: 1011,
    deltaY: 0,
    deltaX: 0,
    heading: 1627,
    deltaZ: 0 }
  t.deepEqual(parsed, expected, 'should parse bitfield')
  t.end()
})


function parse (buf) {
  var parsed =
    { deltaHeading: extract(buf.slice(0), 0, 10)
    , x:            extract(buf.slice(0), 10, 19)

    , y:            extract(buf.slice(4), 0, 19)
    , animation:    extract(buf.slice(4), 19, 10)

    , z:            extract(buf.slice(8), 0, 19)
    , deltaY:       extract(buf.slice(8), 19, 13)

    , deltaX:       extract(buf.slice(12), 0, 13)
    , heading:      extract(buf.slice(12), 13, 12)

    , deltaZ:       extract(buf.slice(16), 0, 13)
    }
    return parsed
}

function extract (buf, offset, length) {
  var full = buf.readInt32LE(0)
  var mask = makeBitMask(length)
  return (full >> offset) & mask
}

function makeBitMask (n) {
  var str = ''
  for (var i = 0; i < n; i++) {
    str += 1
  }
  return parseInt(str, 2)
}

// , /*0094*/ deltaHeading:        vs.Int16LE:10
// , /*----*/ x:                   vs.Int16LE:19
// , /*----*/ padding0054:         vs.Int16LE:3
// , /*0098*/ y:                   vs.Int16LE:19
// , /*----*/ animation:           vs.Int16LE:10
// , /*----*/ padding0058:         vs.Int16LE:3
// , /*0102*/ z:                   vs.Int16LE:19
// , /*----*/ deltaY:              vs.Int16LE:13
// , /*0106*/ deltaX:              vs.Int16LE:13
// , /*----*/ heading:             vs.UInt16LE:12
// , /*----*/ padding0066:         vs.Int16LE:7
// , /*0110*/ deltaZ:              vs.Int16LE:13
// , /*----*/ padding0070:         vs.Int16LE:19

// x = (firstInt >> 10) & 524287 (524287 is 0b1111111111111111111 (19 one's))